概要
Superconducting phenomena in the heavy fermion system UTe2 have attracted much attention [1][2]. This superconductivity exhibits a huge upper critical field along all directions and a tiny decrease of NMR Knight-shift below Tc. These suggest that the Cooper pairs in this superconductivity are spin-triplet pairs. Spin-triplet superconductors have been actively studied because they exhibit unique superconducting phenomena due to the degrees of freedom of spin, and superconductivity in UTe2 is thought to be related to multiple orbitals and to exhibit even more unique superconductivity. In addition, since spin-triplet superconductors are strong candidates for topological superconductors, UTe2 may provide a ideal platform for studying topological properties. Fundamental elements in understanding topological superconducting phenomena are the symmetry of Cooper pairs and the electronic states forming Cooper pairs. The symmetry of Cooper pairs in UTe2 has been investigated by various experiments, including measurements of low-energy excitations of superconducting quasiparticles and spin measurements of Cooper pairs using NMR. However, each experiment has produced different claims, and a clear conclusion has yet to be reached. Regarding the electronic states, recent measurements of the de Haas-Van Alphen effect have reported that UTe2 possesses cylindrical electron and hole Fermi surfaces [3]. In three- dimensional time-reversal symmetric superconductors with cylindrical Fermi surfaces, it is generally known that the topological invariant is trivial. In such cases, considering alternative topological invariants using crystal symmetry becomes effective, leaving the possibility of realizing a topological crystalline superconducting state in UTe2 [4]. In this seminar, I will begin by summarizing recent experimental findings on the superconducting state of UTe2 and then proceed to explain the topological crystalline superconducting state in UTe2.